This guide is about modernist fine art, though many of the suggestions will also be relevant to the study of modernist design.
Summon will usually be your best starting point for finding articles, books and other material on your subject. Watch a video intro to Summon (4 mins) or see our collection of guides to making the most of Summon.
Art & Architecture Source is a major database of journal articles. Not all are in Summon, so it's worth searching separately.
Oxford Art Online is an excellent introduction to artists, schools and themes, in the fine arts in particular. It has several components, of which Grove Art Online and the Benezit Dictionary of Artists are often the most useful. They can be searched separately or together.
Black Artists & Modernism presents an introduction to British modernists of African or Asian descent.